Generally, a radio communication system employs a repeating system if normal services are not provided to terminals because a radio signal transmitted from a base station is weak due to a far distance from the base station, or obstacles such as topographical effects or construction structures, etc., or for service coverage expansion of the base station.
The repeating system typically includes a repeating apparatus having two antennas (a donor antenna and a service antenna) and a PA, wherein the donor antenna communicates signals with a base station and the service antenna communicates signals with terminals.
Further, the repeating system is classified into an optical repeater, an RF repeater, a frequency changing repeater, a microwave repeater and the like depending on how to transmit a signal from a base station to a repeating apparatus. Among these, while the RF repeater is a most cost effective and simple method, it causes oscillations due to a phenomenon, in which a relay signal transmitted through the service antenna is feedbacked and applied to the donor antenna, and thus cannot be normally used.
There are numerous conventional techniques for the RF repeater to remove this feedback interference signal. One of those techniques will be described below with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a bidirectional structure of a conventional RF repeating system.
As shown in FIG. 1, a signal received through a donor antenna 100 (or service antenna 180) is inputted to a filter and signal processor 120 or 160 contained in an RF repeating apparatus 130. Then, the filter and signal processor 120 or 160 removes undesired band components and the signal is amplified by a LNA (Low Noise Amplifier) 140 or 150 prepared at a next stage. The low noise amplified signal is then applied to a duplexer 170 or 110 which is an output amplifier, wherein the output amplifier again amplifies the signal and radiates it in the air through the service antenna 180 or donor antenna 100.
In addition to the above conventional techniques, Korean Patent Application No. 10-2001-0050193 discloses a method for canceling a feedback interference signal by applying a signal processing technique to a received RF signal. Further, Korean Patent Application Nos. 10-2003-0034160 and 10-2005-0127580 improve both flexibility of signal processing and environment adaptation by processing a feedback interference signal using a digital signal processing technique for cancellation thereof in a manner that an RF signal is converted into a baseband signal to remove a feedback interference signal and back into an RF signal for its transmission.
Meanwhile, in the repeating system, an amplifier that amplifies a signal (relay signal) to be transmitted is provided with an LNA and a PA. The main purpose of the LNA is to remove noises. On the other hand, the PA largely amplifies an input signal to provide a high output, rather than removing noises, for high efficiency, but generates much heat. The gain of the PA is defined as a ratio of the amplitude of an input signal to that of an output signal, and is not linearly varied with the amplitude of the input signal but becomes small when the output signal is near to a saturation region.
Further, the difference between the input signal and the output signal of the PA is referred to as distortion. And, when different frequency signals are inputted as input signals, inter modulation occurs between the frequencies, thereby generating, in the output signal, mixed other frequency components that do not exist in the input signal. This is referred to as nonlinear distortion. Otherwise, distortion caused by the amplitude of the signal is referred to as linear distortion. The distortion generated by the PA is classified into an amplitude distortion where amplitude is distorted by the amplitude of the input signal and a phase distortion where phase is distorted by the phase thereof. The nonlinear distortion is greatly generated at a saturation region point where the gain of the PA is maximized. The simplest method for avoiding this is a method that makes the amplitude of the input signal small so that the PA operates enough below the saturation region.
However, if the amplitude of the input signal is small, the efficiency of the PA becomes lower. Meanwhile, in order to improve the nonlinearity of the PA, there have been introduced a feedforward method, a feedback method, a pre-distortion method, etc. As known prior arts, there are Korean Patent Application Nos. 10-2002-0081455, 10-2003-00095905, 10-2002-0031332, 10-2002-0083377, and 10-2003-0007604.
Meanwhile, the non-memory effect is that the output signal of the PA is determined depending on a current input signal only, while the memory (storage) effect refers to characteristic that causes distortion of the PA by nonlinear characteristic thereof by the amplitudes of a signal previously stored in a circuit and a current input signal. The memory effect may be ignored in narrowband where the bandwidth of the input signal is smaller than that of the PA (output amplifier), but becomes very large by frequency differences of input signals in wideband like OFDM (Orthogonal Frequency Division Modulation).
The recent standards for radio communication technologies adopt next generation communication methods using wideband and multi-carriers, and thus, the memory effect greatly appears in the PA. In the feedforward method that has been generally used in the repeating system, the memory effect is offset in course of adding an error signal given by difference between an output signal and an input signal of a primary amplifier. Accordingly, the memory effect does not almost appear, and linearization range is large, and a spurious signal by inter modulation can be effectively removed, so the feedforward method has been widely used in the repeating system up to now.
In case of wideband as set forth above, however, the efficiency of the PA is lower, and the primary amplifier that processes a primary signal and a secondary amplifier that process an error signal are made in an open loop form, thereby making the structure thereof complicated and being sensitive to the characteristic of each device due to necessity of control at the RF end. Especially, with the advancement of semiconductor technology, since the price of high integrated chips and processors such as FPGA (Field Programmable Gate Array) and DSP (Digital Signal Processing) decreases, it becomes possible to implement the baseband pre-distortion technology that processes a baseband signal with the DSP technology.
In order to employ such DSP technology, however, an analog signal of RF band is demodulated into a digital signal and processed by the DSP technology. Then, this is modulated back into an analog signal and inputted to the PA, thereby rendering the implementation thereof complicated and in turn failing to widely use due to price increase by such complicated implementation.
Further, the recent radio communication technology requires the efficient utility of spectrum in order to accommodate many subscribers and support a high communication speed, and thus uses wideband and multi-carrier and a combination of phase and amplitude modulations. In the OFDM that has been recognized as the standard for next-generation radio communication technology, not single sub-carrier but multi sub-carrier is used, so bandwidth is wideband and it exhibits high PAPR (Peak to peak Average Power Ratio) characteristics. Due to this, there are large distortions of the PA by the memory effect and by wideband.
Meanwhile, the distortion of the PA is varied with environments around the repeater, such as deterioration with the lapse of time, temperature, power, and frequency variations by temperature, which causes a large nonlinear phenomenon due to the distortion of the PA. Thus, the PA needs to operate adaptively to various environments.
The conventional linearization technology of the PA set forth above linearizes distortion in order to faithfully amplify an input analog signal, and considers the influence by temperature that is one of causes of the memory effect.
The compensation by such temperature, however, employs a narrow thermal noise compensation technique that reflects temperature around the PA or ambient temperature by the level of input signal, not the narrow PA itself, and thus has the limitations in widespread thermal noise compensation, including ambient radio wave environments, processing gain, system limit, etc. Especially, although the PA is faithfully designed to have linear characteristic, it has the limitations that do not reflect processing limits such as quantization noise caused by the finite of LUT (Look-Up Table), modeling limit, and residual error caused by compromise between the hardware processing speed limit and complexity of implementation, or any error in variations of ambient environments.
Moreover, high output is required to maximize the operation efficiency of the repeating system and a small-sized and lightweight device is required for convenience of installation and maintenance. However, the temperature of PA rises as the output becomes large, and thus the performance of the repeating system is degraded if heat is not effectively radiated to outside, thereby leading to an increase in size and cost due to attachment of a fan for heat radiation.